Re-enable write coalescing

[jacobheun]

Write coalescing was disabled in #24 due to some downstream issues. We need to add more testing, including benchmarks around write coalescing so that we can confidently reenable it.

Success Criteria

• Write coalescing is well tested and benchmarked
• Write coalescing is enabled

[willscott]

ℹ️ Benchmarks can probably be (largely) shared with the go-mplex alternative. Even in that implementation of write-coalescing it's looking like the mechanism is having a performance impact. Ref: libp2p/go-mplex#77